The present invention relates generally to apparatus for performing coherent processing with acoustical waves. More particularly it concerns a novel two-dimensional bulk acoustic wave correlator-convolver that does not require the performance of two Fourier transforms.
It has long been recognized that coherent Fourier processing of picture information and other two-dimensional data displays offers substantial advantages in processing speed because of its inherently parallel, rather than serial, nature. However, the principal disadvantage of most coherent optical Fourier processing systems has been the lack of both adequate interface devices at the input and output of the systems and spatial frequency filters in the interior of the systems.
There has been previously disclosed a coherent acoustic Fourier processing device using bulk acoustic waves that has distinct advantages over its optical counterpart. For a detailed description of this device, see B. A. Auld and D. W. Pettibone, "Processing of Optical Images with Optically Controlled Acoustic Transducers", 1978 Ultrasonics Symposium Proceedings, IEEE Cat. #78CH 1344-ISU, pages 243-249. The use of bulk acoustic waves rather than optics in coherent Fourier processing systems has the advantage that the phase of the distribution at the Fourier transform plane is directly measurable and can be read out serially with an acoustic imaging transducer, thereby facilitating real time phase and amplitude filtering. Like its optical counterpart, such a system requires a high quality spatial modulator at the input.
The aforementioned 1978 IEEE paper describes in great detail a spatial acoustic modulator capable of introducing a desired two-dimensional modulation on a coherent acoustic beam at the input of the system to establish a two-dimensional phase and amplitude modulation of the beam at the Fourier transform plane. To permit use of the system with a serially scanned input and an indirect optical input, such as the electrical input from a TV camera tube, a spatial acoustic modulator is proposed which includes an integrated circuit fabricated with electrodes defining individual transducer elements or cells. High impedance field effect transistors are used to amplify the signals appearing across the transducer elements with additional field effect transistors serving to gate the signals so that the array can be multiplexed.
The work described in the aforementioned 1978 IEEE paper has been extended and is discussed in a subsequent paper by B. A. Auld, D. W. Pettibone, J. D. Plummer and R. G. Swartz entitled "An Electronically Addressed Bulk Acoustic Wave Fourier Transform Device", 1979 Ultrasonic Symposium Proceedings, IEEE Cat. #79CH 1482, pages 184-188. A novel spatial IC-acoustic transducer array was built and tested having the capability of accepting serial electrical data and, through the use of sample and hold units formed on an associated integrated circuit, using the data to spatially modulate a coherent acoustic bulk wave. The Fourier transform of the scanned image appears in the back focal plane of the acoustic lens of the spatial IC-acoustic transducer. The expression "IC" is frequently used herein in place of the term "integrated circuit". Once the Fourier transform of the scanned image is obtained, matched filtering can be derived by a multiplication in the Fourier domain, followed by another Fourier transform.